The present invention relates to PEG-substituted neutral lipopolymers and their use in extended circulating time liposomes. Liposomes containing these lipopolymers provide similar blood circulation times when compared with liposomes incorporating conventional, negatively charged PEG-substituted phospholipids.
Liposomes are used for a variety of therapeutic purposes, and in particular, for carrying therapeutic agents to target cells by systemic administration of liposomal formulations of these agents. Advantageously, liposome-drug formulations offer the potential of improved drug-delivery properties, which include, for example, controlled drug release. An extended circulation time is often needed for liposomes to reach a target region, cell or site. In particular, this is necessary where the target region, cell or site is not located near the site of injection. For example, when liposomes are administered systemically, it is desirable to coat the liposomes with a hydrophilic agent, for example, a coating of hydrophilic polymer chains such as polyethylene glycol, (PEG) to extend the blood circulation lifetime of the liposomes. Such surface-modified liposomes are commonly referred to as xe2x80x9clong circulatingxe2x80x9d or xe2x80x9csterically stabilizedxe2x80x9d liposomes.
The most common surface modification to a liposome is the attachment of PEG chains, typically having a molecular weight from about 1000 daltons (Da) to about 5000 Da, and to about 5 mole percent (%) of the lipids making up the liposomes (see, for example, Stealth Liposomes, CRC Press, Lasic, D. and Martin, F., eds., Boca Raton, Fla., (1995)), and the cited references therein. The pharmacokinetics exhibited by such liposomes are characterized by a dose-independent reduction in uptake of liposomes by the.liver and spleen via the mononuclear phagocyte system (MPS), and significantly prolonged blood circulation time, as compared to non-surface-modified liposomes, which tend to be rapidly removed from the blood and accumulated in the liver and spleen.
The most commonly used and commercially available PEG-substituted phospholipids are based on phosphatidyl ethanolamine (PE), usually DSPE (distearoyl phosphatidyl ethanolamine), which is negatively charged at the polar head group. Negative surface charge in a liposome can be disadvantageous in some aspects, e.g. in interactions with cells (see, for example, Miller et al., Biochemistry, 37:12875-12883 (1998)), and in the delivery of cationic drugs, where leakage of the drug may occur (see, for example, Webb et al., Biochim. Biophys. Acta, 1372:272-282 (1998)).
Accordingly, it would be beneficial to provide uncharged PEG-derivatized lipids that incorporate efficiently into lipid bilayers and provide long blood circulation times to liposomes. Ideally, such lipids are of low toxicity and easily produced.
In one aspect, the present invention includes a liposomal composition having from about 1 mole percent to about 10 mole percent of a neutral lipopolymer wherein the neutral lipopolymer is represented by the formula: 
wherein:
each of R1 and R2 is an alkyl or alkenyl chain having between about 8 carbon atoms and about 24 carbon atoms;
n is between about 10 and about 300,
Z is selected from the group consisting of hydroxy, alkoxy, benzyloxy, carboxylic ester, sulfonic ester, alkyl or aryl carbonate, amino, and alkylamino; and
L is selected from the group consisting of (i) xe2x80x94Xxe2x80x94(Cxe2x95x90O)xe2x80x94Yxe2x80x94CH2xe2x80x94, (ii) xe2x80x94Xxe2x80x94(Cxe2x95x90O)xe2x80x94, and (iii) xe2x80x94Xxe2x80x94CH2xe2x80x94, where X and Y are independently selected from oxygen, NH, and a direct linkage. Preferably, the composition includes from about 3 mole percent to about 6 mole percent of the neutral lipopolymer.
In another aspect, L is a hydrolyzable linkage such as a carbamate linkage, an ester linkage, or a carbonate linkage. In yet another aspect, Z is hydroxy or methoxy. Preferably, R1 and R2 are unbranched. In one aspect, R1 and R2 are both stearyl groups (C17H35). In another aspect, the value of n is preferably between about 20 and about 115, such that the molecular weight of the PEG group is between about 1000 Da to about 5000 Da.
The invention also provides a method for increasing blood circulation time of a liposome containing vesicle-forming lipids, by incorporating in the liposome about 1 mole percent to about 10 mole percent of a neutral lipopolymer having the formula as shown above. The invention further provides lipopolymers represented by this formula.